Various types of adjustable-diameter pulley transmissions have been developed. Often such transmissions take the form of two adjustable pulleys, each pulley having a sheave which is axially fixed and another sheave which is axially movable relative to the first sheave. A flexible belt of metal or elastomeric material is used to intercouple the pulleys. The interior faces of the pulley sheaves are beveled or chamfered so that, as the axially displaceable sheave is moved, the distance between the sheaves and thus the effective pulley diameter is adjusted. The displaceable sheave includes a fluid-constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter; when fluid is exhausted from the chamber, the pulley diameter is changed in the opposite sense. Generally the effective diameter of one pulley is adjusted in one direction as the effective diameter of the second pulley is varied in the opposite direction, thereby effecting a change in the drive ratio between an input shaft coupled to the input pulley and an output shaft coupled to the output pulley. The ratio change is continuous as the pulley diameters are varied, and such a transmission is frequently termed a continuously variable transmission (CVT).
Various refinements were gradually evolved for the hydraulic control system which is used to pass fluid into the fluid-holding chamber of each adjustable pulley. One example of such a hydraulic control system is shown in U.S. Pat. No. 3,115,049 - Moan. The patent shows control of the secondary pulley adjustable sheave, to regulate the belt tension, and a different circuit to regulate fluid into and out of the primary sheave chamber, to regulate the transmission ratio. U.S. Pat. No. 4,152,947 - van Deursen et al also describes control of the CVT. In both systems the line pressure of the fluid applied to hold the belt tension by pressurizing the secondary chamber is relatively high. Thereafter an improved control system was developed to reduce the main line fluid pressure supplied to the secondary sheave chamber as a function of torque demand. This improved system is described and claimed in an application entitled "Control System for Continuously Variable Transmission", Ser. No. 257,035, filed Apr. 24, 1981, which issued June 11, 1985, as U.S. Pat. No. 4,522,086, and assigned to the assignee of this application. Further work resulted in an improved control system which not only reduced the line pressure applied to the secondary sheave chamber to a low, but safe, operating pressure, but also provided a lower control pressure for other portions of the hydraulic control system. This last-mentioned system is described and claimed in an application entitled "Hydraulic Control System for Continuously Variable Transmission", Ser. No. 421,198, filed Sept. 22, 1982, which issued May 7, 1985, as U.S. Pat. No. 4,515,255 and assigned to the assignee of this application.
Additional significant advances in such control systems have been described and claimed in another application also entitled "Hydraulic Control System for Continuously Variable Transmission", Ser. No. 717,913, filed Mar. 29, 1985, and assigned to the assignee of this application. At least some of the improvement achieved in the system of this last-cited application is attributed to the separate and independent control of the fluid actuated clutch. Even with a separate electromechanical control assembly exclusively devoted to clutch control, there remains a need for an effective system and strategy for regulating control of the CVT clutch.